Spark plug and method of making the same



Nov. 2, 1948. T. TOGNOLA ETAL SPARK PLUG AND METHOD OF MAKING THE SAIEFiled July 13, 194:5-

6 6 J 3 um n i f f. V al. 1r /AM f f f f f f f l r /1 /f/ fr ll l f l 1/1/ l /f/f l/lf/l 1 f f f l f f f l. 2 7 7 9 ATTORNEY Patented Nov. 2,

SPARK PLUG AND M ETHOD F MAKING E SAME v Tullio Tognola and Louis H.Segall, Sidney, N. Y., assignor to Bendix Aviation Corporation, NewYork, N. Y., a corporation of Delaware Application July 13, 1943, SerialNo. 494,516

17 Claims. (Cl. 12S-169) This invention relates to ignition apparatusand more particularly to spark plugs adapted for use in igniting thecombustible charges in the cylinders of internal combustion enginesandthe like and to methods of making said spark plugs.

One object of the present invention is to provide a novel spark plugwhich is so constructed that the leakage of gas therethrough from anengine cylinder is wholly or substantially eliminated.

Another object of the invention is to provide a novel radio-shieldedspark plug of simplified construction consisting of a relatively smallnumber of parts.

Still another object is to provide a spark plug structure wherein meansfor reducing or retarding erosion of the electrodes thereof areincorporated in a novel manner.

A further object is to provide a novel center electrode assembly for aspark plug employing ceramic insulation whereby danger of any crackingof the ceramic during assembly or during operation due to widetemperature and pressure changes is reduced to a minimum.

A still further object is to provide an insulated electrode for a sparkplug wherein novel means are utilized for centering the electrode in aninsulator and enhancing the conduction of heat from the electrode ,tothe insulator.

Another object is to provide novel means for securing an electrode inthe bore oi' an insulator fand sealing said bore against the passage ofmoisture and gases.

Another object is to provide a novelly constructed ground electrode in aspark plug whereby the efficient life of the plug may be materiallyenhanced.

The above and further objects and novel features of the invention willmore fully appear from the following detailed description when the sameis read in connection with the accompanying drawings. It is to beexpressly understood, however, that the drawings are for the purpose ofillustration only and are not intended to define the limits of vtheinvention, reference for this latter purpose being primarily had to theappended claims.

In the drawings, wherein like reference characters refer to like partsthroughout the several views,

Fig. 1 is a half-sectional elevation view of one form of radio-shieldedspark plug embodying the present invention;

Fig. 2 is a bottom end view of said spark plug;

Fig. 3 is a side elevation, partly in section, showing yanother form ofspark plug embodying the invention; and

F18. 4 is a bottom view of the spark plug shown in Fig. 3.

'I'he invention is illustrated in the drawings, by way of example, asbeing embodied in radioshielded spark plugs of the general typeuniversally used in present day aircraft engines but it will beunderstood that the features of the invention are equally appli-cable tospark plugs which are not of the shielded type. In the embodiment shownin Fig. 1. the spark plug comprises a tubular met-allie shell I0, whichis externally threaded at II for cooperation with a .threaded opening inan engine cylinder (not shown). The bore through shell'lll is providedwith a tapered wall portion I2 for a purpose which will more fullyappear hereinafter. A ground electrode I3 of any suitable knownconstruction is secured in the lower end of shell IIl by means of silversolder or other suitable means.

Ground electrode I3 is in cooperative spark gap relation with-theenlarged head portion I4 of a center electrode I5 which constitutes apart of novel conducting means mounted in the differential bore I8, Iloi' a ceramic or like insulator I8 for conducting high tensionelectrical current y therethrough. In the form shown, the head I4 of thecenter electrode engages the lower end of lnsulator I8 and the elongatedspindle I9 thereof extends through the small or reduced portion IB ofbore I8, I1 and terminates in the larger portion Il thereof. The upperend of spindle I9 is provided with one or more annular grooves 20 orother suitable deformations for a purpose to appear hereinafter.

In the molding and firing of insulators made of ceramics, aluminum orother metallic oxides and similar insulating materials from whichinsulator I8 may be fabricated, no means or method has yet beendiscovered whereby a ilnished spark plug insulator having exactpredetermined dimensions can be consistently produced with any degree ofcertainty. In view of this fact. the spindles of center electrodes inprior spark plugs, for practical reasons dictated principally byproduction methods and operating conditions, have been slightly smallerthan the bore of the insulator. This has been necessary. for example, toprevent cracking of the insulators due to the difference between thecoefficients of expansion of the metal electrodes and the ceramicinsulators but it induces leakage of gases through the plug. Thisdifference in size often results in a slight longitudinal tilting of theelectrode which. in turn,

3 throws the head I4 thereof out of center with respect to groundelectrode I3, thereby dlslurblng the uniformity of the spark gap betweenthe ad jacent surfaces of head I4 and prongs 2I of the ground electrodeand. hence. reducing the eiliciency and operating life of the sparkplug. Additionally, any space between the spindle and the walls of thebore in the insulator prevents goo'l heat conduction from lthe spindleto the outer cooler portions of the plug, which, in turn, results in theheat being conducted from the hot gases and combustion in the enginecylinder to the lnsulaited incoming or supply cable i not shown), theinsulation of which generally deteriorates rapidly at high temperatures.

The present invention accordingly comprehends means for mounting centerelectrode I5 in insulator I8 in a novel manner whereby accuratecentering of the electrode and good heat conduction .therefrom areassured without subjecting the insulator to the danger of being crackedby expansion of the electrode. In the form illustrated. spindle I8 ofthe electrode has Ithe diameter thereof slightly reduced for a maiorportion of the length thereof in bore ld to form shoulders-I 22, 22. Thediameter of spindle i9 adjacent the ends of said reduced portion, i. e.the diameter of said shoulders, is slightly less than the diameter ofbore I0, the clearance of these parts in the bore being determined bythe accuracy with which insulator I-B is constructed. The surface of thecentral reduced portion of spindle i@ is preferably roughened slightly,such as by sand-blasting, and several layers 23 of thin metallic foil orthe like are wrapped around the same, the number of layers or turnsbeing just suicient to provide a tight siding fit in bore i6.

The wrapping of the foil or similar heat resisting sheet material on thebody of spindle i9 between shoulders 22, 22 and the insertion of thespindle into the bore of the insulator may be eiected by hand whileholding the head itl of the electrode in a tool or chuck, therebyproviding suiiicient leverage or torque to insure tight wrapping oi thefoil and a tight ilt in the insulator. The foil layers 23 serve tocenter the electrode in bore IS and act as a good conductor of heatbetween the electrode and the insulator. Said layers are sufficientlyyicldable. however. in o. radial direction to obviate any danger ofcracku ing of the insulator by expansion of spindle i9 when theassembled parts are subjected to high temperatures during assembly ofthe remainder of the plug and during operation in an engine. Since theheat is conducted away from the nose i6 of the electrode more readilythan in prior spark plugs the pre-ignition pressure of the engine isincreased and a so-called colder" spark plug is obtained.

Novel means are installed in bore il of insulator I8 in a novel mannerfor sealing said bore -against leakage of gases therethrough, forholding electrode I5 in the insulator, and for conducting high tensionelectrical current to said electrode. In the form shown, current isconducted to electrode i5 from a suitable source through a metallicterminal 2, an electrical resistor or other suitable conductor 25 and awire 2S of vz-rf :mail radial cross-section. The space around theseconducting elements is completely lled with a suitable electricallynon-conductive material 2 having a high melting point. The sealing material is a unitary rigid mass and has interlocking relation with spindlei9 at grooves 2o and with terminal 24 at groove 28, or other similar de-4 formations in said spindle and terminal, to thereby hold the parts inassembled relation.

The use of resistor 25 is optional but is preferably provided for thepurpose of reduclngerosion of the active parts I4 and 2I of theelectrodes I3 and I5. Said resistor may have a resistance value ofanywhere from approximately 'l5 ohms to 10,000 ohms, but a value between500 ohms and 1,000 ohms is most desirable for the purpose mentionedabove. One resistor which has been found suitable is made by the Globardivision of the Carborundum Company and is commercially known as Globartype B" resistor. The resistor may be made of the similar material asthe hcating elements for electric furnaces, for example, The ends ofresistor 25 may be protected by metal caps 29 or the like and these, inturn, may be coated with graphite or similar material to insure goodelectrical contact with terminal 24 and conductor 26. vilchougli.'-cvernl suitable non-conducting sealing materials may be employed at2l, one which has been found to be particularly advantageous comprises amixture of approximately five parts by weight of aluminum oxide, such asAloxite cement No. 314 made by the Carborundum Company, and two parts byweight of lead oxide. These proportions are not critical, however. Oneof the considerations is to provide a'sealing material which will becomeplastic-like or pasty only when heated to a temperature above themaximum operating temperature of the spark plug but below anytemperature which might be detrimental to the spark plug parts. Thisresult may be obtained by using a mixture consisting primarily of'aninsulator with a high melting point, such as aluminum oxide, which meltsat approximately 2,050 C., and an yinsulator having a somewhat lowermelting point, such as lead oxide, which melts at approximately 838 C.The aluminum oxide might be replaced in the above mixture by talc(magnesium aluminum silicate), magnesium oxide, bentonite, or otherclays or, in general, any refractory oxide which is adapted to withstandtemperatures comparable to those mentioned above without decomposing.The lead oxide component of the mixture may be replaced by anyinorganic, non-conducting material having a melting point betweenapproximately 900* 1800 F. When a mixture of the above character isheated to a temperature at or slightly above the melting point of thelead oxide or its equivalent, the latter becomes liquid and a pasty orplastic-like compound results which will not become glassy and brittlewhen it solidifies on cooling. The aluminum oxide or equivalentsubstitute gives strength to the mixture when it cools and solidics to aunitary mass.

Conductor 25 may consist of a Nichrome or other suitable wire having asmall diameter. A wire, commercially known as Driver-'Harris Company No.193 alloy having an outside diameter of .008" has been found suitable.One end of said wire is secured to the upper end of spindle I9 beforethe latter is inserted in insulator IB and the free end thereof iscoiled -by winding the same around a suitable tool having a diametersomewhat smaller than that of bore I6. After electrode i5, with wire 26attached thereto, is inserted into insulator I8 in the manner heretoforedescribed, said wire is preerablyfextended until about three turnsthereof remain in bore Il and the same is then cut or substantiallyflush with the end of said insulator.

When the assembling of the parts has reached the stage last mentionedabove, bore I I 'is filled with the sealing material '21 in finelydivided powder forml the components thereof being thoroughly andintimately mixed with one another. A powder which will pass a 200 meshscreen has been found suitable. After the powdered material has beensettled, such as by vibration of the assembly, tamping or the like, thesame is placed in a chuck on a. press and resistor 25 is set on thesurface of the powder in contact with the coiled end of wire 23.Terminal 24 may be magnetically or otherwise suitably supported by theplunger of the press and lowered into engagement with the resistor.While a longitudinal compression pressure of approximately 50 pounds isbeing applied to the parts by the press, one or more gas flames areplayed on the outer surface of insulator I8 or other suitable heatingmeans are employed to raise the temperature of the iilllng powder to themelting point of the lead oxide or other equivalent component of thepowdered mixture having the lowest melting temperature. As the powder isrendered pasty by the melting of one of its components, resistor 25 andterminal 24 will be forced into ilnal position by the applied pressure.When the parts reach the positions illustrated in the drawings, thesource of heat is removed and as soon as the parts have cooled somewhatat normal room temperature7 the center electrode assembly, completedexcept for minor machining operations, is removed from the press formounting in shell I0.

The center electrode assembly, when thus sealed against the leakage ofgases, is preferably mounted in shell i0 in such a manner as to preventany gas leakage between insulator i8 and said shell. This mounting iseffected by wedging a sleeve 3G of relatively soft metal. suchas copper,between a surface of insulator I8 that is coated with a thin layer ofpowdered graphite or the like and tapered wall I2 in shell I0, saidsleeve being held within the shell by a shielding barrel assemblyconsisting of a metallic barrel 3i surrounding a rigid insulating sleeve32. Barrel 3l may be silver-soldered, as at 35, or otherwise suitablysecured in shell Il). Annuler shims 33 made of mica, asbestos or thelike may be interposed between insulating sleeve 32 and metallic sleeve30 to insure proper longitudinal dimensions of the spark plug. For thepurpose of centering sleeve 32 in barrel 3|, any space between theseparts is preferably taken up by one or more layers 34 of sheet asbestosor the like wrapped around said insulating sleeve.

In assembling the insulated center electrode, shell l0 and shieldingbarrel assembly 3|, 32, the center electrode assembly is looselyinserted into shell or body i0, the latter being threaded into orotherwise suitably held by a fixture adapted to position the centerelectrode assembly in proper axial relation to said shell. Copperbushing or sleeve 3D is inserted over insulator I8 into shell I0 betweentapered surface I2 and the outer graphite coated surface of insulatorI8. In order to avoid any possibility of the insulated center electrodebeing dislodged by pressure within an engine cylinder, the outer surfaceof insulator I8 engaged by sleeve 30 is preferably provided with aslight taper. A taper of .010" over the whole length of the insulator issufficient, the smallest diameter being at the upper or outer endadjacent terminal 24. The inner diameter of sleeve 30 is preferablysomewhat greater than the outer diameter of insulator i8 to facilitateassembly and, hence, rests initially upon tapered surface I2 of shellI0.

The shielding assembly 3I, 32. 33 is inserted 6 in the upper, enlargedvbore of shell I0 after the adjacent surfaces of barrel 3| and shell ,I0have been coated with a suitable soldering flux and a ring of silversolder or its equivalent is then placed on the upper end of shell I 0 inclose proximity to the base of barrel 3l. The central portion ,of theentire assembly comprising sleeve 30 and the lower end of barrel 3I isthen heated, such as by means of a Lepel coil or gas flames, to or abovethe melting temperature of the silver solder. l. e.. approximately 1,300F. Electric induction heating as eected by a Lepel coil is preferablesince it localizes the heat and heats both the internal and externalparts quickly with substantial uniformity.

While the parts are thus heated to effect the melting of the solder 35to thereby join shell I0 and barrel 3|, a pressure of approximately1,500

' lbs. is applied to the top of shielding assembly 30,

3 I, thus forcing the latter against the soft metallic sleeve or bushing30 and causing the latter to wedge iirmly between the tapered wall I2 ofshell I0 and the slightly tapered outer surface of. insulator I8. Thisinsures substantially perfect sealing against gas leakage around thecenter electrode assembly and locks the ceramic insulator in positionrelative to shell I0, said position being determined by the shellholding fixture which fixes the shell and center electrode assemblyagainst relative axial movement during the application of pressure tosleeve Sil. The pressure is maintained after termination of the heatinguntil the parts have cooled suiilciently to permit solidii'lcation ofsolder 35 so that it will hold the parts in assembled relation. Byreason of the fact that the bodily expansion of sleeve or bushing 30 isrelied upon to lill the tapered annular space between surface I2 ofshell l0 and insulator I8, it will be seen that exact dimensions for theparts are not essential,

The assembly oi the spark plug is completed by installing groundelectrode i3 in shell ill in spark gap relation with the head I4 ofcenter electrode .I5. The ground electrode may be secured in the shellby means of silver solder or any other suitable means known in the art.

If desired. the annular space between insulator I 8 and insulatingsleeve 32 may be filled with a suitable insulating compound 36. Suitablecompounds for this purpose may consist of polymerized cashew-nut-shelloil or polymerized linseed oil mixed with a suitable drying agent whichwill cause the polymerized oil to solidify through chemical reaction.Such mixtures may be inserted under vacuum in liruid form or the samemay be poured into the bore of shielding assembly 3 I, 32 and thensubjected to pressure. Solidication may be accelerated by application ofheat.

The structure of the spark plug embodying the present invention may bemodied in accordance with the illustration of Fig. 3 wherein the shellI0' and shielding barrel 3l are formed integrally as a unitary element31. In assembling this modified structure, the necessary pressure forwedging sleeve 3U into place may be accomplished by extending a forcetransmitting sleeve or the like through barrel 3| into engagement withsleeve or bushing 30. Thereafter, insulating sleeve 32', 34 and anynecessary shims 33', 33 are inserted in barrel 3| and the outer end of`the latter is spun inwardly to form a retaining flange 38. Shims 33' mayconsist of mica or asbestos washers or other equivalent material.

tures and pressures to 'which the same is subjected during operation inan engine are materially reduced and wherein the deterioration of theelectrode surfaces is reduced to a minimum. Also. this novel spark plugis fabricated from a small number of simple and rugged parts andembodies novel means for mounting and. sealing the center electrodeassembly in the spark plug shelll There is also provided a novel centerelectrode assembly for a spark plug wherein the bore of the insulator issealed against gas leakage therethrough in a novel manner.

Although only two modifications of the invention are illustrated anddescribed, it is to be expressly understood that the same is not limitedthereto. For example, it will now be apparent to those skilled in theart that the invention is equally as applicable to unshielded sparkplugs and other types of shielded spark plugs as it is to the specifictypes shown. It will also be apparent that materials other than thosespecifically mentioned may be used for sealing purposes and for thepurpose of centering certain parts with respect to others withoutdeparting from the invention. Various changes may also be made in thedesign and arrangement of parts illustrated without departing from thespirit and scope of the'invention. For a deiinition of the limits of theinvention, reference is had primarily to the appended claims.

What is claimed is:

1. A spark plug comprising a cylinder engaging shell, a ground electrodesecured in said shell, a center electrode assembly comprising a rigidtubular insulator, a metallic conducting spindle in said insulator, saidspindle terminating at one end intermediate the ends of said insulatorand having an enlarged head portion at the other end thereof in sparkgap relation with said ground electrode and in engagement with alaterally extending surface of said insulator, radially yieldable means,such as a plurality of layers of metallic sheet material, surrounding aportion of said spindle for centering resistor having a resistance inexcess of 75 ohms and a wire of small diameter, a tapered sleeve ofdeformable metal wedged between said insulator and a tapered internalwall of said shell for supporting said electrode assembly in said shell,and means secured in said shell for holding said sleeve againstlmovement out of said shell, said last-named means comprising a metallicshielding barrel and an insulating liner therefor, said liner includinga rigid insulating tube placed end to end with at least one slightlyyieldable insulating washer anda filling of sheet material for centeringsaid rigid ltube in said barrel.

2. A spark plug comprising a cylinder engag.

ing shell, a ground electrode securedv in said shell, and a centerelectrode assembly in said shell comprising a rigid tubular insulator, ametallic conducting spindle in said insulator, said spindle terminatingat one end intermediate the ends of said insulator and having an i spacebetween 7s enlarged head portion at the otherend thereof in spark gaprelation with said ground electrode and in engagement with a laterallyextending surface of said insulator, radially yieldable means, such as aplurality of layers of metallic sheet material, surrounding a portion ofsaid spindle for centering the same in the bore of said insulator, ametallic terminal extending into the bore of said insulator, a rigidmass of non-conducting filling material comprising aluminum and leadoxides in the bore vof said insulator between said terminal and spindle,said mass having interlocking relation with said terminal and spindle,and conducting means embedded in said mass for electrically connectingsaid terminal and spindle, said conducting means including an electricalresistor having a resistance in excess of ohms and a conducting wire ofsmall diameter.

3. A spark plug comprising a tubular shell, a ground electrode securedin said shell, and a center electrode assembly in said shell comprisingan insulating tube, an electrically conducting spindle extending intosaid tube and having lone end thereof in spark gap relation with saidelectrode, said spindle terminating intermediate the ends of said tube,a terminal member extending into said tube in spaced relation with saidspindle, means including a `flne wire for electrically connecting saidspindle and terminal member and a rigid mass of electricallynonconductive material sealing the bore of said tube around said wireand having interlocking relation with 'said spindle and terminal memberfor holding the same in assembled relation.

4. In a spark plug, a tubular insulator, a center electrode in saidinsulator with one end thereof terminating intermediate the ends of saidinsulator, means surrounding a portion of said electrode for centeringthe same in the bore of said insulator, a terminal at the end of saidinsulator in` spaced relation with said end of said electrode, meansforelectrically connecting said terminal and said electrode comprising anelectrical conductor of small transverse crosssection,` and a rigid massof electrically nonconductive materials surrounding said conductor andhaving interlocking relation with said terminal and electrode.

5. In a spark plug, a tubular insulator, a center electrode in saidinsulator with one end thereof terminating intermediate the ends of saidinsulator, means surrounding a portion of said electrode for centeringthe same in the bore of said insulator, a terminal at the end of saidinsulator in spaced relation with said 4end of said electrode, means forelectrically connecting said terminal and said electrode comprising anelectrical conductor of small transverse crosssection, and a rigid massof electrically nonconductive materials surrounding said conductor andhaving interlocking relation with said terminal and electrode, saidconnecting means also including an electrical resistor having a value inexcess of '15 ohms embedded in said rigid mass.

6. In a spark plug, a tubular insulator, current conducting meansextending through the bore of said insulator including a metallicelectrode having an elongated spindle in said bore, and one or morelayers of thin flexible, electrically conductive, heat transmitting,compresy sible sheet material surrounding a portion of said spindle andsubstantially lling the annular b said portion and the wall of said ore.

7. In a spark plug, a rigid tubular insulator, a rigid metallicconductor extending through at least a portion of the bore of saidinsulator, and means for centering said conductor in said borecomprising a plurality of layers of metallic foil surrounding saidconductor.

8. In a spark plug, a rigid insulator having a bore therethrough, meansfor conducting electrical current through said bore, including anelectrode having a head portion disposed externally of said insulatorand an elongated spindle of a diameter less than the diameter of saidbore, said spindle having a reduced portion intermediate the endsthereof forming spaced annular shoulders thereon, and electricallyconductive means comprising a plurality of layers i of metallic sheetmaterial surrounding the reduced portion of said spindle and disposedbetween said shoulders i'or centering said head portion with respect tosaid bore.

9. In a spark plug, an insulator having a bore therethrough, means forconducting electrical current through said bore, including an electrodehaving a head portion and an elongated spindle, and means surroundingsaid spindle for centering said head portion with respect to said bore,said last-named means comprising a plurality of layers of metallic foilsurrounding said spindle within said bore.

10. In a spark plug, a tubular insulator, means for conductingelectrical current through the bore of said insulator, including a rigidmetallic spindle and a plurality of layers of metallic sheet materialsurrounding said spindle for a substantial portion of its length withinsaid bore for centering said spindle in said bore.

11. In a spark plug, a tubular insulator, current conducting ,meansextending through the bore of said insulator including a rigid metallicspindle, and radially-yieldable electrically-conductive means comprisinga plurality of layers of metallic materialin the clearance space arounda portion of said spindle within said bore for centering said spindle insaid bore.

12. In a spark plug, an insulator having a bore therethrough, aconducting spindle secured in said bore Vwith one end thereofterminating intermediate the ends of said insulator, radially yieldablemeans-surrounding said spindle for centering the latter in said bore. aterminal at the end of said insulator in spaced relation with said endof said spindle, means for electrically connecting said terminal andspindle, and a rigid mass of electrically non-conducting`material'sealing said .bore around said connecting means and havinginterlocking relation with said spindle and terminal for holding thesame in assembled relation.

13. A spark plug comprisinga tubular shell.

a ground electrode secured in said shell, and a center electrodeassembly mounted in said shell and comprising an insulating tube, anelectrical- 1y conductive spindle terminating intermediate v the ends ofthe bore of said tube and having one necting means and havinginterlocking relation with said spindle and terminal member for holdingthe same in assembled relation, said wire and a portion of saidnon-conductive mass completely lling an appreciable longitudinal portionof the bore in said tube. 14. The method of making a spark plug whichincludes the steps of making a tubular insulator and a metallicelectrode having an elongated spindle, wrapping a plurality of layers ofmetallic sheet material around at least a portion of said spindle,inserting said Wrapped portion of said spindle in the bore of saidinsulator with said sheet material in close contact with the walls ofsaid bore and securing said spindle in said bore.

15. The method of making a spark plug which includes the steps ofmolding a tubular insulator, making a metallic electrode having aspindle, wrapping a plurality of layers of metallic 'sheet materialaround at least a portion of said spindle, securing a ne, electricallyconductive Wire to one end of said spindle, inserting said spindle inthe bore of said insulator with said sheet material in close contactwith the Walls of said bore and with said wire extending from one end ofsaid bore, iilling the bore of said insulator around said wire and aportion of said spindle with a powdered mixture of insulating materialshaving different melting temperatures, heating said mixture to atemperature equal to at least the melting temperature of the componentof said mixture having the lowest melting temperature, and

inserting electrically conductive terminal means into said bore incontact with said wire to embed at least a portion of said terminalmeans in said insulating mixture, and thereafter permitting 86 saidmixture to cool and solidify.

16. A spark plug comprising a shielding tube, asbestos sheeting snuglyiitting inside the tube, a rigid insulator snugly iitting within theasbestos sheeting, insulation snugly fitting inside D the insulator,another insulator snugly tting within the insulation, and an electrodewithin the last named insulator and snugly tted therein by meanscomprising a winding of iiexible material,

REFERENCES CITED The following references are of record in the file ofthis-patent:

. UNITED STATES PATENTS Number Name Y Date 1,292,661 Stevens Jan. 28,1919 1,588,489 Mosler June l5, 1926 1,812,438 Hurley June 30, 19312,045,781. Kasarjian June 30, 1936 2,060,219 Kasarjian Nov. 10, 19362,079,561 Nowosielski May 4, 1937 2,301,686V Doran Nov. 10, 1942

